In packet-switched communication service systems, digital information is divided into a plurality of information packets which are distributed through a communication network from a sending party to a receiving party. Typically, the information packets are distributed using a plurality of communication paths between the sending party and the receiving party, and the original information is reconstructed at the receiving party from the received information packets. Although the information packets may be distributed using different communication paths, the sending party and the receiving party can be said to be connected by a virtual communication link. By setting up such a virtual communication link, a communication session for sending streamed media data will be established.
In this description, a client terminal is receiving streamed media over a virtual communication link. However, the client terminal and the server, respectively, may be any type of communication equipment suitable for transmitting or receiving streamed media on any type of communication link. For instance, the client terminal can be a mobile terminal in a UMTS (Universal Mobile Telecommunications System) or a computer connected to a computer communication network, etc.
With reference to FIG. 1, a procedure where a client terminal 100 is setting up a communication session for streamed media data with a server 102, according to prior art will now be schematically described. In a first step 1:1, the client terminal 100 initiates a communication session for streamed media data by sending a request to the server 102. Typically, the request is a <SETUP> message, according to known standards, and is therefore not necessary to describe in more detail here. In a following step 1:2, the server 102 confirms and accepts the request by sending an <OK> message to the client terminal 100. Then in a further step 1:3, the client terminal 100 sends another request to the server 102, to initiate the sending of media data from the server 102 to the client terminal. Typically, the start command is a <PLAY> message. In a subsequent step 1:4, the server 102 confirms and accepts the request to send media data, by sending another <OK> message to the client terminal 100. In a final step 1:5, the server starts sending the streamed media data from the server 102 to the communication terminal 100. The command messages are conventional messages according to standards for streamed media data services, which are not described in detail here. Typically, the command messages are supplemented with various information parameters. However, to simplify the understanding of the process, these information parameters are not shown in the schematic FIG. 1.
In general, it is necessary to provide session description information before the session can be executed. The term “session description information” refers to information regarding e.g. available communication paths between a sending party and a receiving party, information capacity for the communication paths, etc. Session description information is typically stored both at the server and at the client terminal.
For distribution of session description information to client terminals in packet-switched communication service systems two methods are generally applied today: Either the session description information will be distributed from the server during the establishment of the communication session, or prior to the establishment of the communication session.
Distribution of session description information prior to establishment of a communication session can be performed when a client terminal wants to access a communication server at a later occasion, such as when a person subscribes to a service which allows downloading of streamed music from a server at a later occasion. The session description information can then be obtained via a communication path or via an alternative communication channel, such as an HTTP connection. An advantage of distributing the session description information prior to establishment of a communication session is that the time for establishing the communication session will be shortened.
Distribution of session description information during establishment of a communication session can be performed by beginning the establishment of the communication session by sending a request for session description information to the server, and then in response receiving the session description information. An advantage of distributing the session description information during the establishment of a communication session is that the session description information is up-to-date.
However, there are certain problems associated with the existing solutions outlined above. The user of a client terminal who wants to establish a communication session for streamed media data has to decide between spending a relatively long time on establishing the communication session, or running the risk of using session description information that is out of date. Using out of date session description information can give rise to unexpected errors if a selected communication path does not exist. On the other hand, the client terminal might not have knowledge of all available communication paths.
The session description information might be corrupted or modified during the transmission from the server to the client terminal, or during the storage at the client terminal.